1. FIELD OF THE INVENTION
This invention relates to calcination of preheated pulverous, raw material, such as cement raw meal, consisting of or containing lime. The invention relates to an improved method of at least partially calcining such pulverous, raw materials and improved calcination plants for treating such raw materials according to the improved method wherein heat is supplied directly by maintaining a flame in a flame chamber to effect at least a partial calcination before the material is subjected to any finishing calcination and/or other heat treatment, if any.
2. Description of the Prior Art
Calcination of pulverous raw materials such as cement raw meal is to be understood as an expulsion of carbon dioxide from calcium carbonate by an endothermic process (i.e. a process in which heat is absorbed) according to the equation: EQU CaCo.sub.3 .fwdarw. Ca0 + CO.sub.2
when the raw material is cement raw meal, a finishing heat treatment following the calcination is a sintering by which cement clinker is produced. Sintering is an exothermic process characterized by, or formed with, evolution of heat.
The heat necessary for carrying through the conversion of cement raw meal to cement clinker is usually obtained by burning fuel which together with combustion air, is introduced into a combustion chamber in which the fuel burns successively with the combustion air and forms smoke gas. As a result, the energy contained in the fuel is released for heating the smoke gas to a high temperature. The hot smoke gas is then brought into contact with the raw meal to be heat treated, i.e. preheated, calcined and burned. The heat treatment usually takes place as a continuous process in a rotary kiln with slightly inclining axis. If so, the lower lying end of the rotary kiln is -- as is usual for rotary kilns -- designed as a combustion chamber.
However, in the manufacture of cement the trend is to carry through preheating and calcination of the raw meal separately by means of hot gas from one source of heat and sintering of the raw meal in a rotary kiln by means of hot gas from another source of heat, that is the flame of the rotary kiln. This is particularly due to the presence of alkalies in the raw meal.
As far as the calcination of cement raw meal is concerned relatively large quantities of materials have to be subjected to a relatively low heating. That is, the process takes place at a rather low temperature which is often inconsistent with the use of smoke gases of a high temperature for carrying through the calcination. There is a great risk of excessively heating the raw meal locally and temporarily. Even a brief period of excessively heating a part of the raw meal may cause expulsion of alkali vapours or produce melts which may involve caking problems. Furthermore, excessive heating of the raw meal at the calcination stage may prevent chemical reactions intended for a later stage of the whole process of manufacturing cement clinker. For example, the formation of clinker minerals at such stage of the whole heat treatment process at which calcination is to take place involves an unfavorable and disadvantageous development of the whole process.
U.S. Pat. No. 3,203,681 to Rosa, et al. relates to a process wherein heat for carrying through the calcination of preheated cement raw meal derives from hot gases having a temperature higher than the calcination temperature. The gases are produced in a separate chamber and are passed upwardly in a riser column in which the raw material is suspended and entrained by the gases thus produced. British Patent No. 960,863 to Fritzsche relates to a process for the calcination of raw silicate material in a fluidised state and the pre-heating, calcination, and sintering, or melting steps, of the heat treatment are effected as separate successive operations in a cyclone pre-heater, a fluidised layer and a combustion chamber respectively. However neither of these patents disclose or suggest a method of heat treating a pulverous, raw material or a plant for practicing the method such as I have invented as will develop in the following description. According to my invention, a raw material consisting entirely of, or at least containing a portion of, lime is at least partially calcined substantially isothermically (i.e. constant temperature conditions) at relatively low temperatures by supplying the heat for carrying out the calcination or partial calcination at the place where the raw materials are present, suspended in a gas, and maintained in a fluidised accumulation.